Method and system for controlling amplifier power in an optical communications network having add/drop capability

ABSTRACT

The invention includes a method for controlling amplifier output power in an optical communications network having channel add/drop capability. A first transmission parameter and a second transmission parameter are determined at a first amplifier. In an exemplary embodiment, the first transmission parameter is a composite express signal-to-noise ratio and the second transmission parameter is a composite signal-to-noise ratio. The total output power of a downstream amplifier is adjusted in response to the first transmission parameter and second transmission parameter. A system for implementing the method is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. provisionalapplication serial No. 60/289,672, filed May 9, 2001, the entirecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The invention relates generally to a method and system forcontrolling amplifier power in an optical communications network.

[0004] 2. Description of Related Art

[0005] Wave length division multiplexing (WDM) has been used to increasethe capacity of existing fiber optic networks. In a WDM system, pluraloptical signal channels are carried over a single optical fiber witheach channel being assigned a particular wavelength. Such systemstypically include a plurality of receivers, each detecting a respectivechannel by effectively filtering out the remaining channels.

[0006] Optical channels in a WDM system are frequently transmitted oversilica based optical fibers, which typically have relatively low loss atwavelengths within a range of 1525 nm to 1580 nm. WDM optical signalchannels at wavelengths within this low loss “window” can be transmittedover distances of approximately 50 km without significant attenuation.For distances beyond 50 km, however, optical amplifiers are used tocompensate for optical fiber loss.

[0007] Optical amplifiers have been developed which include an opticalfiber doped with erbium known as erbium-doped fiber amplifiers or EDFAs.The erbium-doped fiber is “pumped” with light at a selected wavelength,e.g., 980 nm, to provide amplification or gain at wavelengths within thelow loss window of the optical fiber. Other types of optical amplifiersinclude erbium-doped waveguide amplifiers (EDWA), semiconductor opticalamplifiers (SOA).

[0008] When optical amplifiers are cascaded in series along atransmission span, noise generated at each amplifier degrades the signalto noise ratio. FIG. 1 is a block diagram of a conventional opticalcommunications network having a plurality of amplifiers 10 ₁, 10 ₂ and10 ₃ positioned along transmission fiber 12 ₁, 12 ₂ and 12 ₃. The outputof each amplifier includes noise in the form of amplified spontaneousemissions (ASE) and at least one signal as shown in FIG. 1. It isunderstood that a WDM system may carry multiple signals on separatechannels and a single signal is shown for ease of illustration.

[0009] As is known in the art, at each amplifier stage the ASE increasesdue to the amplification of ASE input to the amplifier and ASE added atthe amplifier. If, however, the output power of the amplifiers 10 ₁, 10₂ and 10 ₃ are equal, then the signal component is decreased toaccommodate for the increase in ASE. For example, if the total outputpower of each amplifier is 8 mw, the power available for the signal isreduced as the ASE power increases from one amplifier to the next. Asshown in FIG. 1, setting the output power of each amplifier equalresults in a decreased signal-to-noise ratio (SNR) as the signal passesthrough multiple amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

[0011]FIG. 1 is a block diagram of a portion of a conventional opticalcommunications network;

[0012]FIG. 2 is a block diagram of a portion of an opticalcommunications network in an embodiment of the invention;

[0013]FIG. 3 is a block diagram of a portion of an opticalcommunications network in an alternate embodiment of the invention;

[0014]FIG. 4 depicts the effect of adding and dropping channels alongthe optical communications network;

[0015]FIGS. 5A and 5B illustrate two different techniques for droppingchannels;

[0016]FIG. 6 is a flowchart of a method of controlling amplifier powerin a first embodiment of the invention;

[0017]FIG. 7 is a flowchart of a method of controlling amplifier powerin a second embodiment of the invention; and

[0018]FIG. 8 is a flowchart of a method of determining channel power inan exemplary embodiment of the invention.

SUMMARY OF THE INVENTION

[0019] The invention includes a method for controlling amplifier outputpower in an optical communications network having channel add/dropcapability. A first transmission parameter and a second transmissionparameter are determined at a first amplifier. In an exemplaryembodiment, the first transmission parameter is a composite expresssignal-to-noise ratio and the second transmission parameter is acomposite signal-to-noise ratio. The total output power of a downstreamamplifier is adjusted in response to the first transmission parameterand second transmission parameter. A system for implementing the methodis also disclosed.

[0020] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. Also, the following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims and equivalents thereof.

[0022] The expression “optically communicates” as used herein refers toany connection, coupling, link or the like by which optical signalscarried by one optical system element are imparted to the“communicating” element. Such “optically communicating” devices are notnecessarily directly connected to one another and may be separated byintermediate optical components or devices. Likewise, the expressions“connection” and “operative connection” as used herein are relativeterms and do not require a direct physical connection.

[0023]FIG. 2 is a block diagram of a portion of an opticalcommunications network in an embodiment of the invention. The opticalcommunications network provides communications between transmitter 80and receiver 82. In an exemplary embodiment, the optical communicationssystem is a WDM system in which the transmitter 80 generates a pluralityof signals, each on a separate wavelength or channel. Receiver 82detects the individual channels as known in the art.

[0024] The optical communications network includes a number ofamplifiers 50 ₁, 50 ₂ and 50 ₃ which may be implemented using EDFAs. Theamplifiers 50 serve as signal regeneration points along sections oftransmission fiber 52 ₁, 52 ₂, 52 ₃. It is understood that othercomponents may be included in the transmission span such as switches,couplers, etc.

[0025] Each amplifier 50 is in communication with a communicationsdevice 62 and an associated node control processor (NCP) 64. NCP 64 maybe a microprocessor-based controller executing a computer program todetermine the total output power for one or more amplifiers 50. It isunderstood that a one-to-one correspondence between amplifiers 50 andNCPs 64 is not required. An NCP 64 may interface with a plurality ofamplifiers 50 or a single master NCP may interface with all amplifiers.The communications device 62 may be implemented using a service channelmodem (SCM). Service channel modems may provide for communicationsbetween network elements as described in further detail in U.S. Pat.Nos. 6,163,392, 5,978,115 and 5,532,864, the entire contents of whichare incorporated herein by reference. The NCPs 64 monitor transmissionparameters at each amplifier 50 and communicate with amplifiers 50 andother NCPs 64 via communication devices 62.

[0026] In an exemplary embodiment, the NCPs communicate over a servicechannel defined by one or more wavelengths (e.g., 1625 nm) carried ontransmission fiber 52. The communications devices 62 use this wavelengthto communicate without interfering with the signals carried on separatewavelengths. Alternatively, the NCPs may communicate over anothercommunications network separate from transmission fiber 52.

[0027] In operation, the NCPs 64 adjust the output power at eachamplifier 50 so that the SNR for each channel remains substantiallyconstant over the entire transmission span. Each NCP shares transmissionparameters with other NCPs to adjust the output power at each amplifier.In an exemplary embodiment, each NCP 64 stores a network profileidentifying the network elements in the transmission span. Thus, eachNCP 64 knows its position in the communications network along with othertransmission parameters such as the number of channels input and outputat each amplifier, SNR at each amplifier, etc. Two exemplary embodimentsof the invention are described herein. With reference to FIG. 2, in afirst embodiment it is assumed that the number of channels transmittedalong the transmission span does not vary. Thus, if 16 channels are sentby transmitter 80, the same 16 channels are received at receiver 82. Inthe second embodiment described with reference to FIGS. 3-5, add/dropmultiplexers 54 are positioned in the transmission span to provide theability to add and/or drop channels.

[0028] Referring to FIG. 2, each NCP 64 adjusts the total output powerof a corresponding amplifier 50 based on at least one transmissionparameter obtained from an upstream amplifier. The term “upstream”refers to a direction opposite the transmitter-receiver path. The termdownstream refers to a direction consistent with thetransmitter-receiver path. At each amplifier location, the NCP 64determines the signal output power and the composite express outputpower. These values are used to determine the total output power for theamplifier. The signal output power may be calculated based on channelcount, channel plan information, and power adjustments (e.g., fiber typeadjustment, user-defined power adjustment). The composite express outputpower may be determined by the sum of the signal output power, theupstream ASE power and the current amplifier ASE power. Alternatively,one or both of the signal output power and the composite express outputpower can be measured.

[0029] From the above output power values, a composite expresssignal-to-noise ratio (snr_ratio) may be calculated at each NCP 64 whichis communicated to downstream NCPs (and optionally upstream anddownstream NCPs) in the transmission span via communication devices 62.The composite express signal-to-noise ratio at an amplifier output maybe calculated by dividing the signal output power by the compositeexpress output power. The composite express output power may becalculated based on knowledge of the immediate upstream amplifier'scomposite express signal-to-noise ratio. The upstream composite expresssignal-to-noise ratio may be defined as 1 for the first amplifier (e.g.,amplifier 50 ₁) in a transmission span.

[0030] A derivation of the calculation performed to determine theappropriate output power at an amplifier is provided below. Thefollowing variables are used in the derivation.

[0031] Pin_(j): Total Input Power of amp j (not including powerassociated with communication device 62).

[0032] Pout_(j): Total Output Power at amp j (not including powerassociated with communication device 62).

[0033] Pout_express_(j): Total Output Power at amp j assuming no blocks(not including power associated with communication device 62).

[0034] Psig_in_(j): Total Signal Input Power at ampj=Psig_out_(j-1)/Loss_(j)

[0035] Psig_out_(j): Composite Signal Output Power at amp j.

[0036] Gain_(j): Gain of ampj=(Psig_out_(j)/Nout_(j))/(Psig_in_(j)/Nout_(j))

[0037] Loss_(j): Fiber Loss from output of j-1 amp to input of amp j=10^(Fiber Loss_(j)/10)

[0038] Snr_ratio_(j): Ratio of the composite signal power to totaloutput power at Amp j calculated as if there were no added/droppedchannels in the transmission span.

[0039] Pase_express_(j): Total ASE power at output of Amplifier jassuming no blocks

[0040] Pase_(j): Total ASE power at output of Amplifier j.

[0041] Padded_ase_(j): Added ASE power at Amplifier j. This ratio may bedefined at the output of the amplifier.

[0042] Equation (11) below is used to determine the total output powerPout_express₁ for the i^(th) amplifier so that the signal-to-noise ratioalong the transmission span remains substantially constant. Equation(11) is derived as follows.

[0043] First, the signal output power is defined as shown in equation(1) where Padj₁ is a user-defined power adjustment factor for the i^(th)amplifier.

Psig _(—) out _(i) =linearValue(REF1)*10^ (Padj _(i)/10)  (1)

[0044] The quantity linearValue(REF 1) designates a signal output powerbased on a predetermined channel power multiplied by the number ofoutput channels. In an exemplary embodiment, this value is 0.6 mwmultiplied by the number of output channels. As described in furtherdetail herein, the channels may be weighted to define channel units inwhich case the signal output power is the per channel unit powermultiplied by the number of channel units. Defining channel power basedon channel units provides more accurate power allocation. A channel isassigned a number of channel units based on transmission factors such astransmission rate and transmission format. The power allocated thatchannel is then determined based on the number of channel units.

[0045] The power adjustment value Padj_(i) is a user-defined poweradjustment. The amplifiers are specified to output the signal at somepredefined level (e.g., −2 dBm per channel). The power adjustment valuePadj_(i) allows the user to adjust this level. For example, the poweradjustment value may be 1 dB causing the amplifier to output the signalat −1 dBm per channel.

[0046] The total output power is represented as shown in equation (2).As can be seen, the total output power is a sum of the signal outputpower Psig_out_(i), ASE power from the preceding (i−1^(th)) amplifierwhich is amplified at the i^(th) amplifier and any ASE power added atthe i^(th) amplifier.

Pout _(—) express _(i) =Psig _(—) out _(i) +Pase _(—) express_(i−1)*(Gain _(i) /Loss _(i))+Padded _(—) ase _(i)  (2)

[0047] where: Pase₀=0.

[0048] The composite express signal-to-noise ratio (snr_ratio_(i)) maybe defined as the ratio of output signal power to the total outputpower. Again, with reference to FIG. 2, it is assumed that no add/dropmultiplexers (ADMs) are located in the transmission span. This providesa direct measure of the added ASE power between two amplifiers in thetransmission span. The composite express signal-to-noise ratio isdefined as:

snr _(—) ratio _(i) =Psig _(—) out _(i) /Pout _(—) express  (3)

=Psig _(—) out _(i)/(Psig _(—) out _(i) +Pase _(—) express _(i−1)*(Gain_(i) /Loss _(i))+Padded _(—) ase _(i)).  (4)

[0049] The (Pase_express_(i)*(Gain_(i)/Loss_(i))) term may berepresented using the following:

Pase _(—) express _(i−1) =Pout _(—) express _(i−1) −Psig _(—) out_(i−1)  (5)

=Psig _(—) out ⁻¹*(1/snr _(—) ratio ⁻¹−1)  (6)

Psig _(—) out _(i−1) =Psig _(—) in _(i) *Loss _(i)  (7)

Gain _(i)=(Psig _(—) out _(i) /Nout _(i))/(Psig _(—) in _(i) /Nin_(i)).  (8)

[0050] Inserting equations (7) and (8) into equation (6) gives:

Pase _(—) express _(i−1)*(Gain _(i) /Loss _(i))=(Nin _(i) /Nout_(i))*Psig _(—) out _(i)*(1/snr _(—) ratio _(i−1)−1).  (9)

[0051] Inserting equation (9) into equation (2) yields:

Pout _(—) express _(i) =Psig _(—) out _(i)*(1+(Nin _(i) /Nout_(i))*(1/snr _(—) ratio _(i−1)−1))+Padded _(—) ase _(i).  (10)

[0052] In the embodiment shown in FIG. 2, the number of channels acrossthe transmission span remains constant so Nin_(i) and Nout_(i) areequal. Thus, equation (10) reduces to

Pout _(—) express _(i) =Psig _(—) out _(i) /snr _(—) ratio _(i−1)+Padded _(—) ase _(i)  (11)

[0053] The power attributable to ASE added at the i^(th) amplifier maybe derived as follows:

Padded ^(—) ase _(i)=10log(F _(ase)(gain)*((Psig _(—) out _(i) /Nout_(i))/((Pin _(current) _(—) _(val) /Nin _(i))*sig _(—ratio)_(i−1))+1)*6.1334*10⁴).

[0054] F_(ase)(gain) is the ASE gain for the i^(th) amplifier. Thisvalue may be retrieved from a table indexed by the per-channel gain ascalculated by

10log((Pout _(current) _(—) _(val) /Nout _(i))/((Pin _(curret) _(—)_(val) /Nin _(i))*sig _(—) ratio _(i−1))+1)

[0055] where Psig_out=calculated signal output power level of amplifieri in mW

[0056] Pin_(current) _(—) _(val)=measured input power level (currentvalue) of amplifier i in mW

[0057] Nin_(i)=number of input channel at amplifier i

[0058] Nout_(i)=number of output channel at amplifier i.

[0059] In the embodiment of FIG. 2, the number of input channels equalsthe number of output channels at each amplifier along the transmissionspan. As described in further detail herein, each channel may beweighted to define channel units in which case the signal output poweris the per channel unit power multiplied by the number of channel units.Defining channel power based on channel units provides more accuratepower allocation.

[0060] It can be seen from equation (11) that the composite expresssignal-to-noise ratio from the i−1^(th) amplifier is used to determinethe output power for the i^(th) amplifier. It is understood thattransmission parameters other than composite express signal-to-noiseratio may be used to determine the output power of downstreamamplifiers. For example, the channel count N may be used by downstreamamplifiers to establish the signal power.

[0061] The calculation of output power in equation (11) does not includepower used to provide a service channel for the communications devices62 if present on the transmission fiber 52. This service channel for thecommunications devices 62 may be outside the amplification band ofamplifiers 50 and thus is not included in equations (1)-(l 1). Anadditional power offset (e.g., 2 mw) may be added to the output powerdetermined in equation (11) to provide for the service channel utilizedby communications devices 62.

[0062] The service channel utilized by communications devices 62 ispreferably an “out-of-band channel”, meaning that this communicationschannel is distanced from the channels carrying the transmissionsignals. In an exemplary embodiment, the out-of-band service channel is1625 nm. This wavelength is preferable because it is not affected byamplifiers 50 and may still be utilized even if amplifiers 50 areinoperative.

[0063] To illustrate determination of amplifier output power, referenceis made to FIG. 2. In an exemplary embodiment, NCP 64 ₁ sets the outputpower of amplifier 50 ₁ based on equation (11). Because amplifier 50 ₁is the first amplifier in the transmission span, the value of theupstream composite express signal-to-noise ratio (snr_ratio₀) is setto 1. NCP 64 ₁ determines the composite express signal-to-noise ratio(snr_ratio₁) and broadcasts this transmission parameter to at least theimmediate downstream NCP 64 ₂, if not multiple upstream and downstreamNCPs.

[0064] At amplifier 50 ₂, NCP 64 ₂ determines the appropriate outputpower based on equation (11). The value for the composite expresssignal-to-noise ratio (snr_ratio₁) has been received from NCP 64 ₁. NCP64 ₂ determines the composite express signal-to-noise ratio (snr_ratio₂)and broadcasts this value to at least the immediate downstream NCP 64 ₃,if not all NCPs. Typically, the output power of each successiveamplifier is increased as the signal proceeds along the transmissionspan to maintain the signal-to-noise ratio for each channelsubstantially constant.

[0065] An exemplary process of controlling the output power of anamplifier, where no channels are added or dropped, is depicted in FIG.6. At step 100, the first transmission parameter (e.g., compositeexpress signal-to-noise ratio) is determined for a first amplifier andthis transmission parameter is broadcast to other network elements atstep 102. At step 104, signal output power for the second amplifier isdetermined and at step 106, amplified spontaneous emission generated atthe second amplifier is determined. The proper amount of output powerfor the second amplifier is determined at step 108 is response to thefirst transmission parameter, the signal output power for the secondamplifier and amplified spontaneous emission generated at the secondamplifier. This determined value is then used to control output power ofthe second amplifier at step 110.

[0066] In an alternate embodiment of the invention shown in FIG. 3, thecommunication network has the ability to add and/or drop channels alongthe transmission span. This may be accomplished through an add/dropmultiplexer 54 which may be implemented using an optical add/dropmultiplexer (OADM). Alternatively, amplifiers 50 may include componentsto provide the add/drop function.

[0067] When channels can be added and/or dropped, the ASE may vary foreach channel depending on whether the channel has been added or droppedand the type of drop. A drop where a section of the bandwidth isattenuated (including ASE and signal) and thus prevented from continuingdown the span is referred to as a block. Such a block may beaccomplished through gratings, filters, etc. The block substantiallyeliminates all power (ASE, signal, etc) for a section of bandwidthdownstream of the OADM. The degree of attenuation is limited by theefficiency of the component performing the block.

[0068] For example, reference is made to FIG. 3 which depicts a singlesignal being carried on one channel. The signal and ASE are depictedbelow each corresponding amplifier 50. Assume that prior to amplifier 50₃ at add/drop element 54 ₃, the signal is blocked (e.g., delivered to arecipient) and a new signal is added on the same channel. As shown inFIG. 3, the ASE associated with the newly added signal is zero. Thus,the determination of amplifier output power will vary based on whetherand where in the transmission span channels were added and/or blocked.

[0069] The block-type drop is contrasted with a drop-and-continue. Whena drop-and-continue is performed, the signal and ASE are dropped from afirst transmission path and routed to a second transmission path withoutattenuation. Thus, in a drop-and-continue, the downstream amplifier inthe second transmission path does not need to compensate for a reductionin ASE.

[0070] The NCPs adjust output power of a respective amplifier toaccommodate for ASE power reduction due to blocked channels. Adownstream amplifier determines the amount of ASE blocked by upstreamnetwork elements. This determination may be performed by receivingtransmission parameters (e.g., signal-to-noise values, channel counts,etc.) from upstream elements and determining the amount of ASE powerblocked upstream. Alternatively, ASE power may be measured at pointsalong the transmission path. Based on the amount of ASE power blockedupstream, the downstream amplifier output power is adjusted accordingly.

[0071] In an exemplary embodiment, each NCP determines signal outputpower, composite express output power and composite output power.Alternatively, one or more of these powers may be measured. The signaloutput power and composite express output power are determined asdescribed above with reference to FIG. 2. The composite output power isthe amplifier output power taking into account the effects of ASE powerbeing blocked by one or more ADMs upstream in the transmission span.

[0072] From these three power values, a composite expresssignal-to-noise ratio and a composite signal-to-noise ratio aredetermined. The composite express signal-to-noise ratio (snr_ratio) isdetermined as described above with reference to FIG. 2. The compositesignal-to-noise ratio (sig_ratio) at an amplifier output is calculatedby dividing the signal output power by the composite output power. Thecomposite express signal-to-noise ratio (snr_ratio) and the compositesignal-to-noise ratio (sig_ratio) may be calculated at each NCP 64 andthen communicated to downstream NCPs (and optionally all NCPs) in thetransmission span.

[0073] In the embodiment of FIG. 3, each NCP 64 determines amplifiertotal output power based on the upstream amplifier's compositesignal-to-noise ratio (sig_ratio) as well as the composite expresssignal-to-noise ratio (snr_ratio) at all upstream amplifier's at ADMsites. The upstream composite signal-to-noise ratio (sig_ratio) and thecomposite express signal-to-noise ratio (snr_ratio) used by the firstamplifier in a transmission span are implicitly defined as 1.

[0074] A derivation of the calculation performed to determine the totaloutput power at an amplifier, including adjustments for adds/blocks, isprovided below. In addition to the variables described above withreference to FIG. 2, the following variable is used in the derivation.

[0075] Sig_ratio_(j): Ratio of Composite Output Signal Power to TotalOutput Power at Amp j. This ratio may be defined at the output of theamplifier.

[0076] Equation (22) below is used to determine the proper signal outputPout_(i) for the i^(th) amplifier so that the signal-to-noise ratio foreach channel along the transmission span remains substantially constant.Equation (22) is derived as follows.

[0077] The output power at the i^(th) amplifier is first established asshown in equations (12)-(14).

Pout _(i)=Total output power (not including communication device 62power) at amp _(i)  (12)

sig _(—) ratio _(i) =Psig _(—) out _(j) /Pout _(j)  (13)

Pout _(i) =Psig _(—) out _(i) +Pase _(i−1)*(Gain _(i) /Loss_(i))+Padded_ase_(i).  (14)

[0078] Performing the same substitutions as in equations (5) through (9)above, except for defining:

Pase _(i−1) =Pout _(i−1) −Psig _(—) out _(i−1)  (15)

=Psig _(—) out _(i−1)*(1/sig _(—) ratio _(i−1)−1)  (16)

[0079] yields:

Pase _(i−1)*(Gain _(i) /Loss _(i))=(Nin _(i) /Nout _(i))*Psig _(—) out_(i)*(1/sig _(—) ratio _(i−1)−1).  (17)

[0080] Substituting equation (17) into equation (14) and combiningterms, yields:

[0081]Pout _(i) =Psig _(—) out ₁*(1+(Nin _(i) /Nout _(i))*(1/sig _(—)ratio _(i−1)−1))+Padded _(—) ase _(i).  (18)

[0082] If the number of input channels (Nin_(i)) and the number ofoutput channels (Nout_(i)) are the same at an amplifier, then equation(18) is reduced to:

Pout _(i) =Psig _(—) out _(i) /sig _(—) ratio _(i−1) +Padded _(—) ase_(i).  (19)

[0083] Adjusting equation (19) for added and blocked channels provides:

Pout _(i) =Pout _(i)−Σ_(ch)(Pase _(—) express _(i−1)*(Gain _(i) /Loss_(i))−Pase ⁻ express _(k))*filt_bw_(ch).  (20)

[0084] Subscript ch designates each channel on the current amplifierthat has been blocked. Variable Pase_(k) is the total ASE power at theoutput of amplifier “k” which is the amplifier positioned after thefirst upstream element blocking channel ch. The subscript “k” is usedwith variables corresponding to the first amplifier upstream ofamplifier “i” where channel ch has been blocked. For example, referringto FIG. 4, amplifier “k” is amplifier 50 ₁ which is positionedimmediately after the first upstream element, ADM 54 ₁, blocking channelch. Variable filt_bw_(ch) is a variable dependent on the type of blockused in the ADM.

[0085] Equation (20) allows the proper amount of ASE to be subtractedfrom the amplifier total output power. FIG. 4 depicts the subtraction ofASE provided by equation (20) for determining the output power atamplifier 50 ₃. Assume that prior to amplifier 50 ₁, a channel isblocked (e.g., delivered to a recipient) and added back in (e.g., a newsignal is submitted) at ADM 54 ₁. At amplifier 50 ₁, the ASE for thatchannel substantially zero. As the signal passes through amplifier 50 ₂,the ASE is increased for all channels, including the added channel. AtADM 54 ₃, the channel is again blocked and added to place the ASE powerfor that channel substantially equal to zero. The output power of theamplifier 50 ₃ needs to be compensated to reflect the loss of ASE powerfor this channel between the output of amplifier 50 ₂ and the input ofamplifier 50 ₃.

[0086] To determine the amount of ASE power lost, equation (20) dictatesthat the amplified ASE power from the previous amplifier,Pase_express_(i−1)*(Gain_(i)/Loss_(i)), is reduced by the ASE power atthe first network element where the channel was blocked,Pase_express_(k). Relating to FIG. 4, this corresponds to subtractingthe ASE power at amplifier 50 ₁ from the ASE power at amplifier 50 ₂.This determines the amount of ASE power (shown cross-hashed in FIG. 4)reduced through the block at ADM 54 ₃.

[0087] The ASE power adjustment variable filt_bw_(ch) adjusts the amountof ASE power subtracted due to blocked channels. The ASE poweradjustment variable may take on different values depending on the typeof channel block that is employed. As shown in FIG. 5A, a first type ofblock used in the art is a channel-by-channel block. This type of blockattenuates the signal and associated ASE in individually, spacedchannels. Thus, some ASE power remains un-blocked between the blockedchannels. A second type of block is a band block in which an entirerange of wavelengths is removed. As shown in FIG. 5B, all signals andall associated ASE within a band are removed.

[0088] Referring to equation (20), if a channel-by-channel block isemployed, then the ASE power adjustment variable filt_bw_(ch) has avalue smaller than if a band block is used. This is due to the fact thatmore ASE power should be subtracted when a band block is used. In anexemplary embodiment, the ASE power adjustment variable filt_bw_(ch) isabout 0.005 if a channel-by-channel block is used and about 0.010 if aband block is used. It is understood that other values may be useddepending on a variety on network characteristics such as channelspacing, etc.

[0089] In the second embodiment, the number of signals can vary from oneamplifier to the next given the add/drop functionality. The signal powerat the network element where the channel was first blocked isrepresented as

Psig _(—) out _(k) =Psig_out_(i)*(Nout_(k)/Nout_(i))*(10^ (Padj_(k)/10)/10log(Padj _(i)/10)).  (21)

[0090] Nout_(k) and Nout_(i) represent the number of channels output atthe amplifier where the channel was first blocked and the currentamplifier, respectively. As described in further detail herein, thenumber of channels may be represented as channel units to effectivelyweight the channels and accurately allocate power to each channel.Padj_(k) and Padj_(i) are user-defined power adjustment factors (e.g.,measured in dB) for the amplifier where the channel was first blockedand the current amplifier, respectively.

[0091] Substituting equations (6), (9), and (21) into equation (20)yields equation (22) as follows:Pout_(i) = Psig  _  out_(i) * (1 + (Nin_(i)/Nout_(i)) * (1/sig  _  ratio_(i − 1) − 1)) + Padded  _  ase_(i) − Σ_(c  h)Psig  _  out_(i) * ((Nin_(i)/Nout_(i)) * (1/snr  _  ratio_(i − 1) − 1) − (Nout_(k)/Nout_(i)) * (10log (Padj_(k)/10)/10log (Padj_(i)/10)) * (1/snr  _  ratio_(i − 1) − 1)) * filt  _  bw_(ch).

[0092] As evident from equation (22) and the above description, in thesecond embodiment, the power at the i^(th) amplifier is dependent on afirst transmission parameter (composite signal-to-noise ratio sig_ratio)and a second transmission parameter (composite express signal-to-noiseratio snr_ratio) from the i−1^(th) amplifier. The output power at thei^(th) amplifier is also dependent on whether channels have been blockedalong the transmission span and the type of block (band or channel)performed through ASE power adjustment variable filt_bw_(ch). Thus, thenumber of channels output at each amplifier is broadcast to the NCPs 64.As described above, additional power (e.g. 2 mw) may be added to theoutput power determined in equation (22) to provide for the channelutilized by communications devices 62. Preferably, this communicationschannel is an out-of-band channel as described above with reference toFIG. 2.

[0093] An exemplary process of controlling the output power of anamplifier, where channels may be added or blocked, is depicted in FIG.7. At step 200, the first transmission parameter (e.g., compositeexpress signal-to-noise ratio) and second transmission parameter (e.g.,composite express signal-to-noise ratio and composite signal-to-noiseratio) are determined for a first amplifier. These transmissionparameters are broadcast to other network elements at step 202. At step204, signal output power for the second amplifier is determined and atstep 206, amplified spontaneous emission generated at the secondamplifier is determined. The output power for the second amplifier isdetermined at step 208 is response to the first transmission parameter,the second transmission parameter, the signal output power for thesecond amplifier and amplified spontaneous emission generated at thesecond amplifier. This determined value is then used to control outputpower of the second amplifier at step 210.

[0094] The channel count variable N used in the above equations may bedependent upon transmission parameters such as transmission rate andtransmission format of the signal. A channel may be weighted as multiplechannel units depending on transmission rate and transmission format.FIG. 8 is a flowchart of a process for determining the number of channelunits corresponding to channel. The method may be implemented by an NCP64 in response to a computer program in a storage medium accessible bythe NCP.

[0095] Referring to FIG. 8, the process begins at step 300 where atleast one transmission parameter is determined for a channel.Transmission parameters may relate to transmission rate (e.g., bps) ortransmission format (e.g., the use of forward error correction). At step302, the number of channel units for the channel are determined. Thedetermination of the number of channel units may be based on any numberof transmission parameters. Once the number of channel units isdetermined, the power for the channel is determined at step 304. Thechannel power may be determined based on the number of channel unitsmultiplied by a power per channel unit value. The channel power can thenbe controlled as shown at step 306.

[0096] In an exemplary embodiment, a signal having a transmission rateof 2.5 Gbps is weighted as 1 channel unit. A signal having a 10 Gbpstransmission rate using forward error correction is weighted as 2channel units. A signal having a 10 Gbps transmission rate withoutforward error correction is weighted as 4 channel units. The presence ofhigher weighted channels will increase the power requirements of theamplifiers. Table A below depicts exemplary channel units and theassociated channel power for different transmission rates andtransmission formats. The variable×represents a power per channel unitvalue. TABLE A Channel Transmission Transmission Number Rate FormatChannel Units Channel Power 1 2.5 Gbps FEC 1 1x 2 10 Gbps FEC 2 2x 3 10Gbps non-FEC 4 4x

[0097] The processing performed to determine the total output power maybe implemented by a microprocessor-based NCP. Thus, the invention may beembodied in the form of computer program code containing instructionsembodied in tangible media, such as floppy diskettes, CD-ROMs, harddrives, or any other computer-readable storage medium, wherein, when thecomputer program code is loaded into and executed by a computer, thecomputer becomes an apparatus for practicing the invention. Alsoincluded may be embodiments in the form of computer program code, forexample, whether stored in a storage medium, loaded into and/or executedby a computer, or as a data signal transmitted, whether a modulatedcarrier wave or not, over some transmission medium, such as overelectrical wiring or cabling, through fiber optics, or viaelectromagnetic radiation, wherein, when the computer program code isloaded into and executed by a computer, the computer becomes anapparatus for practicing the invention. When implemented on ageneral-purpose microprocessor, the computer program code segmentsconfigure the microprocessor to create specific logic circuits.

[0098] The invention being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas departure from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method of controlling amplifier output power inan optical communications network, the method comprising: for a firstamplifier, determining a first transmission parameter associated withthe output power of the first amplifier and a second transmissionparameter associated with the output power of the first amplifier; for asecond amplifier downstream from the first amplifier; determining asecond signal output power for the second amplifier; determining anamount of amplified spontaneous emission generated at the secondamplifier; and determining an output power for the second amplifier inresponse to the second signal output power, the first transmissionparameter, the second transmission parameter and the amount of amplifiedspontaneous emission generated at the second amplifier; and controllingoutput power of the second amplifier in response to the determinedoutput power for the second amplifier.
 2. The method of claim 1 whereinthe first transmission parameter for the first amplifier is a compositeexpress signal-to-noise ratio.
 3. The method of claim 2 wherein thecomposite express signal-to-noise ratio is determined by: determining afirst signal output power for the first amplifier; determining acomposite express output power for the first amplifier; and determiningthe composite express signal-to-noise ratio in response to the firstsignal output power and the composite express output power.
 4. Themethod of claim 1 wherein the second transmission parameter for thefirst amplifier is a composite signal-to-noise ratio.
 5. The method ofclaim 4 wherein the composite signal-to-noise ratio is determined by:determining a first signal output power for the first amplifier;determining a composite output power for the first amplifier in responseto added and blocked channels; and determining the compositesignal-to-noise ratio in response to the first signal output power andthe composite output power.
 6. The method of claim 1 further comprisingbroadcasting the first transmission parameter and the secondtransmission parameter to downstream amplifiers in the opticalcommunications network.
 7. The method of claim 1 further comprisingbroadcasting the first transmission parameter and the secondtransmission parameter to upstream and downstream amplifiers in theoptical communications network.
 8. The method of claim 1 wherein saidcontrolling output power of the second amplifier includes adding a poweroffset to provide for a service channel.
 9. A method of controllingamplifier output power in an optical communications network, the methodcomprising: for a first amplifier: determining a first signal outputpower for the first amplifier; determining a composite express outputpower for the first amplifier; determining a composite output power forthe first amplifier in response to added and blocked channels;determining a composite express signal-to-noise ratio in response to thefirst signal output power and the composite express output power;determining a composite signal-to-noise ratio in response to the firstsignal output power and the composite output power; for a secondamplifier downstream from the first amplifier; determining a secondsignal output power for the second amplifier; determining an amount ofamplified spontaneous emission generated at the second amplifier; anddetermining an output power for the second amplifier in response to thesecond signal output power, composite express signal-to-noise ratio,composite signal-to-noise ratio and the amount of amplified spontaneousemission generated at the second amplifier; and, controlling outputpower of the second amplifier in response to the determined output powerfor the second amplifier.
 10. An optical communications networkcomprising: a first amplifier; a processor associated with said firstamplifier, said processor associated with said first amplifierdetermining a first transmission parameter associated with the outputpower of the first amplifier and a second transmission parameterassociated with the output power of the first amplifier; a secondamplifier downstream from said first amplifier; a processor associatedwith said second amplifier, said processor associated with said secondamplifier: determining a second signal output power for said secondamplifier; determining an amount of amplified spontaneous emissiongenerated at said second amplifier; and determining an output power forsaid second amplifier in response to the second signal output power, thefirst transmission parameter, the second transmission parameter and theamount of amplified spontaneous emission generated at said secondamplifier; and controlling output power of said second amplifier inresponse to the determined output power for said second amplifier. 11.The network of claim 10 wherein the first transmission parameter forsaid first amplifier is a composite express signal-to-noise ratio. 12.The network of claim 11 wherein the composite express signal-to-noiseratio is determined by: determining a first signal output power for saidfirst amplifier; determining a composite express output power for saidfirst amplifier; and determining the composite express signal-to-noiseratio in response to the first signal output power and the compositeexpress output power.
 13. The network of claim 10 wherein the secondtransmission parameter for said first amplifier is a compositesignal-to-noise ratio.
 14. The network of claim 13 wherein the compositesignal-to-noise ratio is determined by: determining a first signaloutput power for said first amplifier; determining a composite outputpower for said first amplifier in response to added and blockedchannels; and determining the composite signal-to-noise ratio inresponse to the first signal output power and the composite outputpower.
 15. The network of claim 10 wherein said processor associatedwith said first amplifier broadcasts the first transmission parameterand the second transmission parameter to downstream amplifiers in theoptical communications network.
 16. The network of claim 10 wherein saidprocessor associated with said first amplifier broadcasts the firsttransmission parameter and the second transmission parameter to upstreamand downstream amplifiers in the optical communications network.
 17. Thenetwork of claim 10 wherein said processor associated with said secondamplifier controls output power of said second amplifier by adding apower offset to provide for a service channel.
 18. The network of claim10 wherein said processor associated with said first amplifier and saidprocessor associated with said second amplifier are the same processor.19. The network of claim 10 wherein said processor associated with saidfirst amplifier and said processor associated with said second amplifierare separate processors.
 20. An optical communications networkcomprising: a first amplifier; a processor associated with said firstamplifier, said processor associated with said first amplifier:determining a first signal output power for said first amplifier;determining a composite express output power for said first amplifier;determining a composite output power for said first amplifier inresponse to added and blocked channels; determining a composite expresssignal-to-noise ratio in response to the first signal output power andthe composite express output power; determining a compositesignal-to-noise ratio in response to the first signal output power andthe composite output power; a second amplifier; a processor associatedwith said second amplifier, said processor associated with said secondamplifier: determining a second signal output power for said secondamplifier; determining an amount of amplified spontaneous emissiongenerated at said second amplifier; and determining an output power forsaid second amplifier in response to the second signal output power,composite express signal-to-noise ratio, composite signal-to-noise ratioand the amount of amplified spontaneous emission generated at saidsecond amplifier; and, controlling output power of said second amplifierin response to the determined output power for said second amplifier.21. The network of claim 20 wherein said processor associated with saidfirst amplifier and said processor associated with said second amplifierare the same processor.
 22. The network of claim 20 wherein saidprocessor associated with said first amplifier and said processorassociated with said second amplifier are separate processors.
 23. Amethod of determining amplifier power for an amplifier in an opticalcommunications network, the method comprising: determining an amount ofASE power blocked at a network element upstream of the amplifier; and,determining output power for the amplifier in response to the amount ofASE power blocked at the network element upstream of the amplifier. 24.The method of claim 23 wherein said determining the amount of ASE powerblocked at the network element upstream of the amplifier includesreceiving a transmission parameter indicative of the amount of ASE powerblocked at the network element upstream of the amplifier.
 25. The methodof claim 24 wherein the transmission parameter includes a compositeexpress signal-to-noise ratio.
 26. The method of claim 24 wherein thetransmission parameter includes a composite signal-to-noise ratio. 27.An optical communications network comprising: an amplifier; and aprocessor associated with said amplifier, said processor: determining anamount of ASE power blocked at a network element upstream of saidamplifier; and, determining output power for said amplifier in responseto the amount of ASE power blocked at the network element upstream ofthe amplifier.
 28. The network of claim 27 wherein said processorreceives a transmission parameter indicative of the amount of ASE powerblocked at the network element upstream of said amplifier.
 29. Thenetwork of claim 28 wherein the transmission parameter includes acomposite express signal-to-noise ratio.
 30. The network of claim 28wherein the transmission parameter includes a composite signal-to-noiseratio.